The present invention relates generally to motion control systems.
In the following discussion, an image scanner is used as an example of a system where the invention may be used, but the invention is applicable to many other motion control systems.
A scanner produces a digital representation of an original item. A variety of originals may be scanned, including documents, photographs, transparencies, or three-dimensional objects. The scanner maps locations on the original to memory locations, usually in a computer file.
The scanner typically sweeps a scanning mechanism near the original. The mechanism may contain a light source for illuminating the original, optical components for creating an image of the original, and sensors for converting the image to electronic signals. The scanning mechanism is typically actuated by a drive system that may include a motor, gears, belts, pulleys, cables, or other components.
Most often, one of two motor types is used. Many scanners use a stepper motor. A stepper motor moves its shaft angularly in response to the magnitudes and directions of currents in two or more winding phases. As the winding currents are changed, the motor shaft moves to different equilibrium positions, and thus the motor shaft position may be controlled by controlling timing, magnitudes, and directions of the winding currents. Stepper motors are often driven xe2x80x9copen loopxe2x80x9d. That is, no measuring device is used to provide feedback as to the motor shaft position. As long as the motor is driven within its operating envelope, it can be counted on to move its shaft, and consequently the scanning mechanism, to the commanded positions. Stepper motors provide fine control of the scanning mechanism position, especially when reduction gears are used in the drive system, and thus can allow construction of scanners with very high resolutions. However, stepper motors often have a limited speed range over which they can operate. Using reduction gears to increase the scanning resolution further reduces the speed at which the scanner can operate with a given stepper motor.
Some scanners use a DC motor drive. A DC motor provides a torque in proportion to the current in its winding. It has no inherent positioning means, so an external position-measuring device is used, often an optical encoder. A controller, usually comprising a microprocessor, reads the motor or scan mechanism position and adjusts the motor winding current in such a way as to cause the scan mechanism to sweep through a series of desired positions at the proper times. DC motors may operate at substantial speeds, but the available encoders often cannot provide the resolution attainable with a stepper motor and reduction gears.
Speed and resolution are two important components of scanner performance. Both high speed and high resolution are desired, but these are often competing goals. It is desirable that a scanner be able to scan at high resolution when required and scan at high speed when required. Each motor type has a disadvantage in either speed or resolution.
There is a need for a motion control system that can both provide high resolution and scan at high speed.
A motion control system comprises both a stepper motor and a DC motor. The motors are coupled, and may be operated separately or in concert for various scan types. The stepper motor may be used as an encoder for measuring the position of the mechanism.